Implantable medical devices (IMDs), such as pacemakers, determine whether capture has occurred in response to a stimulation pulse in order to determine the effectiveness of the pacing therapy administered to the patient. The term “capture” generally refers to a cardiac depolarization and contraction of the heart in response to a stimulation pulse applied by the implantable medical device. To determine whether a stimulation pulse is capturing a ventricle, an IMD monitors the cardiac activity of a patient to search for presence of an evoked response following the stimulation pulse. The evoked response is an electrical event that occurs in response to the application of the stimulation pulse to the heart. The cardiac activity of the patient is monitored through the medical device by tracking stimulation pulses delivered to the heart and examining, via one or more electrodes on leads deployed within the heart, electrical activity signals that occur concurrently with depolarization or contraction of the heart.
The evoked response is often difficult to detect due to a pace polarization artifact, which is also referred to as a post-pace polarization artifact or a pace polarization signal. Pace polarization effects or artifacts are present on the sensing electrode employed to sense the electrical activity of the heart. Polarization of the pacing electrode is caused by accumulation of charge on an interface between the electrode and the cardiac tissue of the heart during delivery of a stimulation pulse.
Differentiating between pace polarization artifacts and evoked response signals can be problematic. For example, residual pace polarization artifacts typically have high amplitudes even when evoked response signals do occur. Additionally, a patient exhibiting a fast ventricular rate causes repolarization (i.e. T wave) to be pushed closer to depolarization (i.e. QRS wave). Consequently, differentiating between pace polarization artifacts and evoked response signals becomes even more difficult, if not impossible, using a conventional pacemaker or pacer cardioverter defibrillator (PCD) sense amplifier employing linear frequency filtering techniques. Additionally, the generated polarization artifact may result in the pacemaker identifying a false evoked response, which in turn leads to missed heartbeats. Furthermore, the polarization signal can cause the pacemaker to fail to detect an evoked response that is in fact present.
A variety of techniques have been used to reduce pace polarization artifacts. For example, U.S. Pat. No. 7,089,049 to Kerver et al. is configured to remove polarization artifacts from electrical activity signals in order to improve detection of an evoked response. More specifically, a IMD receives a signal that represents electrical activity within a heart of a patient following delivery of a stimulation pulse to the heart and reconfigures a filter state of a filter from an initial filter state to remove the polarization artifact from the electrical activity signal in order to determine whether a cardiac event, such as an evoked response has occurred. The medical device may, for example, when the filter of the medical device is a digital filter, recalculate the values of digital filter components using the present input value of the electrical activity signal as a direct current (DC) input value of the digital filter. While a digital filter may provide useful results, the filter adds cost to the implantable medical device. It is therefore desirable to develop additional or alternative methods that can be employed to further reduce or avoid polarization effects when detecting distinct waves in a cardiac signal.